This invention relates to an electronic thermostat having a controller for normally controlling the activation of a heating or cooling system. In particular this invention relates to the deactivation of the heating or cooling system when the controller malfunctions.
Heating, ventilation and air conditioning (HVAC) systems are commonly used in homes and commercial establishments to provide temperature controlled environments. Generally, a thermostat is used to manually set a desired temperature for an area heated or cooled by the HVAC system. The thermostat includes a controller which cooperates with a temperature sensor that senses the temperature of the area being heated or cooled. If the temperature has deviated from the desired temperature set on the thermostat, the controller will send a signal which will turn on the HVAC system.
In many instances the controller will be a microprocessor that receives signals from the temperature sensor and electronically controls the operating state of the HVAC system. However, these microprocessors do not always operate perfectly and field conditions may occasionally cause the microprocessor to malfunction. Thus, if a microprocessor controlling a furnace accidently locks up in a state which leaves the furnace on, the area serviced by this furnace can reach extremely high temperatures. Such extreme temperatures can cause damage to plants, pets, and others in the area who may not be able to turn power off to the furnace for whatever reason. Therefore, it is essential to have an accurate and reliable safety cutoff mechanism to protect against such controller lock up situations. One method used to protect against controller lock up situations is to place a bimetallic temperature switch at the output of a relay which controls the on/off state of the HVAC system. When this mechanical safety switch heats up to a certain temperature range, it will send a signal cutting off power to the HVAC system. However, given the mechanical characteristics of the bimetallic temperature switch, this safety device is not always reliable and may fail to work in some instances. In addition, under normal working conditions, the bimetallic temperature switch has a tolerance of approximately plus or minus ten degrees Fahrenheit from the temperature at which it should be activated. Having such a large tolerance in the safety device can be dangerous since it may not allow power to be cutoff to the HVAC system at a safe temperature. Also, this bimetallic temperature switch is expensive to use.